The present invention relates to a exemplary method for demodulating a carrier wave modulated using a digital symbol sequence and transmitted over a noise-impacted channel, the ideal edge shapes of possible transitions between two symbols being known and stored in memory (reference edges), and a received edge being scanned and digitalized using a scanning frequency which is a multiple of the frequency of the symbol sequence.
Continuous phase modulation (CPM) may be used to transmit (data) packets. In this context, by varying the phase angle (relation, position), a plurality of different symbols, e.g., four, can be transmitted. Since modulation using square-wave (rectangular) pulses all having the same symbol length may lead to a very broad spectrum, modulation may be carried out using pulses that extend over the length of two symbol periods and that are represented as cosine-shapedxe2x80x94to avoid steep edges. A modulation of this type is also known as 2RC-CPM.
The conference proceedings,
xe2x80x9cK.-H. Tietgen, xe2x80x98Numerical Modulation Methods Applied in the FD/TDMA System S 900 D,xe2x80x99 Second Nordic Seminar on Digital Land Mobile Radio Communications, Stockholm, 1986,xe2x80x9d
describe numerically modulating edges between two consecutive symbols, rather than to use the symbols themselves for modulation.
FIG. 1 shows the block diagram of a transmitter that is well-suited to use the known method. In this context, the system parts shaded in gray operate in the high system clock.
FIG. 2, for a four-value symbol alphabet, shows the conceivable edges in the transition from one specific symbol to the next in a symbol sequence to be transmitted. The idea in the known modulation methods is to describe the instantaneous frequency (i.e., the phase derived from time) of a (for example) four-step signal given a pulse shape of a length 2T as a side-by-side arrangement (consecutive alignment) of 16 possible edges fN(t) having length T. Instead of transmitting overlapping, shifted elementary pulses xcexa3id(I)g(txe2x88x92iT), is also possible to directly transmit a non-overlapping sequence of edges xcexa3ifN(txe2x88x92iT), neighboring symbols d(I) and d(I+1)T in each case deciding which of the 16 edges in interval iTxe2x89xa6txe2x89xa6(I+1)T is transmitted.
In the cited literature reference, a method of this type is termed CP-4FSK. To simplify the comparison with other methods, in what follows, it will be termed Num2RC 4st, derived from a four-step method, in which modulation takes place numerically and the edge shapes are based on the application of a xe2x80x9craised cosinexe2x80x9d lasting for two symbols.
To receive a data sequence modulated in this way, a receiver has been proposed in the seminar report,
xe2x80x9cD. E. Pfitzmann and H.-P. Ketterling, xe2x80x98A New CP-4FSK Sampling Demodulator for the FD/TDMA System S 900 D,xe2x80x99
Second Nordic Seminar on Digital Land Mobile Radio Communications, Stockholm, 1986.xe2x80x9d
The same receiver is also known from German Published Patent Application No. 36 28 993.
Using the receiver described above, a received edge is scanned and digitalized using a scanning frequency which is a multiple of the frequency of the symbol sequence. In the example cited, scanning is carried out between two symbols, for example, 16 times. The digitalized values are compared with each other and, in the event that the difference is small between two consecutive values, are combined into a symbol average (center). By averaging consecutive values it is attempted to eliminate the influence of noise.
Classical CPM receivers and demodulation methods are also termed xe2x80x9cdiscriminator Integrate and Dump.xe2x80x9d Therefore, in what follows, when a signal edge shape of the xe2x80x9craised cosinexe2x80x9d lasting for two symbol lengths is used, the term 2RCdsIandD is used. In the following, the term may be supplemented by the addition of xe2x80x9c2stxe2x80x9d (i.e., two-step, two symbols) or xe2x80x9c4stxe2x80x9d (i.e., four-step, four symbols).
In general, it is desirable for a receiver if each individual symbol is received in a highly reliable manner. In the classical CPM receiver method, it is necessary to filter the signal that has become noisy in passing through the real transition channel. Conventional filters operate as an integrator, for example, over two symbol periods, to average the noise. In the above-mentioned 2RC signal shapes, integration leads to intersymbol interference, abbreviated as ISI. To avoid intersymbol interference, a Viterbi algorithm may be applied to the received symbol sequence. Disadvantageous in this context is the great computing effort that places correspondingly great demands on the hardware.
Therefore, the exemplary embodiment according to the present invention is based on the objective of indicating a demodulation method in which it is possible to have small computing effort while maintaining the error rate at as close to the same level as possible. In this manner, the design effort for a receiver is simplified without relinquishing the advantages of xe2x80x9cContinuous Phase Modulationxe2x80x9d (CPM) such as low complexity and a compact spectrum, which may be important with respect to good bandwidth utilization.
The objective is believed to be achived through the exemplary embodiment according to the present invention in that, to detect a received and scanned edge, all of the scanning values are used, in each case, to calculate Euclidean distances from at least two reference edges, and the reference edge having the lowest Euclidean distance is selected.
In contrast to the cited classical receivers, in a demodulation method of this type, instead of the symbols to be estimated, the transition edges between the symbols are made central to the detection process. These edges are scanned multiple times and are compared with the original edges. Subsequently, the decision is made in favor of the edge whose distance, in the Euclidean sense, from the received edge is the smallest. This decision is carried out edge-by-edge independently of the neighboring edges, the decision hypothesis therefore not necessarily guaranteeing the continuity of the derivation of the phase of the CPM signal.
In another exemplary embodiment, there is a continuous phase.
Therefore, according to the exemplary embodiment according to the present invention, a viterbi algorithm may be applied to a number of consecutive edges, the respective Euclidean distances between the edges received in one symbol period and the reference edges being considered as the cost of a trellis branch of the Viterbi algorithm.
The properties of the CPM modulation thus make it possible to carry out an equalization of the data signals very efficiently.
The proposed maximum-likelihood estimation of the received data sequences using a Viterbi algorithm not on the symbol sequences but on the received edges has the consequence that a model can be used whose number of states is smaller by a factor of M in comparison to the classical equalizer. In this context, M designates the step-quantity of the signal. Therefore, the computing effort required can be kept small, as a result of which the hardware implementation becomes more attractive, as is described below.